The invention relates to a method for manufacturing a plug-type chamber, which accommodates at least two floating plugs and is delimited on one side by a drawing point and, on the other side, by an indentation applied to the outer surface of a starting tube. Such a method is useful in the cascade drawing of tubes made of non-ferrous metals or their alloys.
The invention further provides a device for implementing the method.
EPO 353 324 B1 discloses providing a plug-type chamber for floating plugs directly in the longitudinal section, behind a drawing point premolded on a starting tube. The plug-type chamber is delimited on its other side by an indentation, which is pressed in radially from the outside into the tubular material. As a result, the freely movable floating plugs are embedded in the plug-type chamber and are not lost. The starting tube can be an extruded tube, or rather a milled or a longitudinal-seam-welded tube.
To reduce the outside diameter and the wall thickness, the starting tube is moved in a plurality of drawing operations through drawing dies, which have progressively smaller opening cross-sections, the floating plugs having different diameters, each forming the corresponding thrust block.
In the disclosed case, in order to properly implement the cascade drawing operation, the wall thickness of the starting tube having a specific outside diameter must be dimensioned so as to enable the floating plug having the smallest diameter to move freely in the plug-type chamber until taking on the thrust-block function, even allowing for the manufacturing tolerances of the input stock, such as irregular outside diameter, uneven wall thickness and ovalness. If this dimensioning is not attended to, the floating plug can become jammed and cause the starting tube to be broken off. During every drawing operation, up until when the drawing die and the floating plug interact, a hollow drawing automatically takes place. During this hollow drawing operation, the inside diameter of the tubular section of the starting tube to be reduced in outside diameter is reduced in size in the area of the plug-type chamber. Therefore, the wall thickness is also slightly increased as a result of the compressing operation upon passing through the drawing die. For this reason, this technique forces one to reduce the wall thickness of the starting tube so as to guarantee the free mobility of the smallest floating plug. However, this results in less material being used and results in the less economical production of finished tubing.
There remains a need for improvements to this method, as well as for the associated structure necessary to practice such an improved method. Such an improved method should allow more material to be employed per starting tube and, accordingly, permit the production of more finished tubing without any loss of quality.